A metal oxide semiconductor field effect transistor (MOSFET) includes a gate insulating layer and a gate electrode sequentially stacked on a semiconductor substrate. To achieve higher operating speeds and to save electric power, complimentary metal oxide semiconductor (CMOS) type devices include both NMOS and PMOS transistors. These CMOS-type semiconductor devices may use the same conductive material for gate electrodes for NMOS and PMOS transistors to reduce fabricating steps. The conductive material used for gate electrodes can be n-type polycrystalline silicon (“polysilicon”) in general and the gate insulating layers are most commonly made of silicon oxide layers.
As semiconductor devices provide higher operating speeds, a thickness of the gate insulating layer may be reduced. If the thickness of the gate insulating layer is reduced to less than a critical thickness, leakage current may occur thereby degrading characteristics of the semiconductor devices. Recently, thicknesses of silicon oxide layers used as the gate insulating layers have approached this critical thickness. Thus, further reductions in thicknesses of silicon oxide layers used as gate insulating layers may be limited. Therefore, use of high-k dielectrics as gate insulating layers have been studied.
High-k dielectrics may provide improved characteristics with respect to reducing leakage currents even when an equivalent oxide thickness (EOT) is less than a critical thickness of a silicon oxide layer. The EOT of a high-k dielectric layer means the thickness of a silicon oxide layer that would provide the same capacitance. Thus, use of a high-k dielectric can provide a capacitance equivalent to that provided using a physically thinner silicon oxide layer while providing improved leakage current characteristics.
On the other hand, if a high-k dielectric is used as a gate insulating layer, the high-k dielectric may react with n-type polycrystalline silicon used as the gate electrode to form a silicon oxide layer. As a result, the silicon oxide layer formed through the reaction may increase a total EOT of the gate insulating layer. In addition, a work function of the materials of the gate electrode may have an effect on electrical characteristics of the transistor.